Recapturing the mojo: AMD’s Radeon HD 7970 GHz Edition

Stress-testing AMD's latest video card.

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In the great game of one-upsmanship played by the two major graphics chip makers, one of the most prized goals is being first to market with a new generation of technology. AMD captured that waypoint late last year when it introduced the first 28-nm GPU, the Radeon HD 7970.

However, there are advantages to being later to market, because the competition has already played its hand. Nvidia smartly took advantage of that dynamic when it unveiled the GeForce GTX 680 several months ago. The new GeForce managed—just barely—to outperform the 7970, while consuming less power and bearing a price tag $50 lower than the Radeon's. Nvidia couldn't have pulled off that trifecta if not for the efficiency of its Kepler architecture, of course, but knowing the target surely helped in selecting clock speeds and pricing for the final product. The first reviews of the GTX 680 were uniformly positive, and the narrative was set: Kepler was a winner. Despite being second to market—or, heck, because of it—Nvidia had captured the mojo.

Then an interesting thing happened. Finding a GeForce GTX 680 card in stock at an online retailer became difficult—and the situation still hasn't eased. Meanwhile, Radeon HD 7900-series cards appear to be plentiful. AMD's spin on this situation is simply to point out that its cards are more readily available for purchase, which is undeniably true. Nvidia's take is that it's selling through GTX 680s as fast as it can get them—and that the problem is raging demand for its products, not just iffy supply. Since both companies rely on the same foundry (TSMC) for their chips, we suspect there's some truth in Nvidia's assertions. These things are hard to know for sure, but quite likely, the GTX 680 is outselling the 7970—perhaps by quite a bit.

If so, that's just a tad insane, given how closely matched the two cards have been in our assessments. Evidently, capturing the mojo is very important indeed.

AMD's answer to this dilemma is a new variant of the Radeon HD 7970 intended to reclaim the single-GPU performance crown, the awkwardly named Radeon HD 7970 GHz Edition. Compared to the original 7970, the GHz Edition has higher core (1GHz vs. 925MHz) and memory (1500MHz vs. 1375MHz) clock speeds, and it has a new "boost" feature similar to Kepler's GPU Boost.

To understand the "boost" issue, we have to take a quick detour into dynamic voltage and frequency scaling (DVFS) schemes, such as the Turbo Boost feature in Intel's desktop processors. AMD was the first GPU maker to introduce a DVFS scheme for graphics cards, known as PowerTune. PowerTune allows AMD to set higher stock GPU clock frequencies than would otherwise be possible within a given thermal envelope. The GPU then scales back clock speeds occasionally for workloads with unusually high demands, to enforce its power limits. Unlike the various Turbo and Boost schemes on other chips, though, PowerTune doesn't raise clock speeds opportunistically in order to take advantage of any extra thermal headroom—at least, it hasn't until now.

Like the Turbo Core feature in AMD's FX processors, PowerTune works by monitoring digital activity counters distributed around the chip and using those inputs to estimate power consumption. These power estimates are based on profiles developed through extensive qualification testing of multiple chips. Somewhat uniquely, AMD claims the behavior of its DVFS schemes is deterministic—that is, each and every chip of the same model should perform the same. Intel and Nvidia don't make such guarantees. If you get a sweetheart of a Core i5, it may outperform your neighbor's; better cooling and lower ambient temperatures can affect performance, as well.

For the 7970 GHz Edition, AMD has refined its PowerTune algorithm to improve its accuracy. By eliminating some cases of overestimation, AMD claims, this revamped algorithm both increases the GPU's clock speed headroom and allows the GPU to spend more time resident at its peak frequency. Furthermore, the 7970 GHz Edition adds an additional P-state that takes the GPU clock beyond its stock speed, to 1050MHz, when the thermal envelope permits. It ain't much in the grand scheme, but this ability to reach for an additional 50MHz is the 7970 GHz Edition's "boost" feature—and it is fairly comparable to the GPU Boost capability built into Nvidia's Kepler.

The higher default clock speeds and the PowerTune wizardry are the sum total of the changes to the GHz Edition compared to the original Radeon HD 7970. GHz Edition cards should still have the same ~250W max power rating, with six- and eight- pin aux power connectors. Above is a picture of our 7970 GHz Edition review unit, which came to us directly from AMD. However, there is a bit of a catch. The card above is based on AMD's reference design, but we understand retail cards from AMD's various partners will have custom coolers and possibly custom PCB designs. You won't likely see a 7970 GHz Edition that looks like that picture.

We'd like to show you a retail card, but those aren't here yet. AMD tells us the first products should begin showing up at online retailers next week, with "wide availability" to follow the week after that.

Here's a look at how the 7970 GHz Edition compares to a couple of Radeon HD 7900 cards already on the market. As you can see, the GHz Edition's higher core and memory clock speeds separate it pretty clearly from the stock 7970 in key rates like pixel fill, texture filtering, shader flops, and memory bandwidth.

In fact, although it's not listed in the table above, the 7970 GHz Edition is the first GPU to reach the 1 teraflop milestone for theoretical peak double-precision floating-point math throughput. Double-precision throughput is irrelevant for real-time graphics and probably mostly useless for consumer GPU-computing applications as well. Still, this card hits a target recently mentioned by both Nvidia and Intel as goals for data-parallel computing products coming later this year.

AMD says the GHz Edition will list for $499.99, placing it directly opposite the GeForce GTX 680. We've taken the prices for the other two Radeons above from Newegg. Street prices for the Radeon HD 7970 have recently dropped to $449.99, 100 bucks below its introductory price, perhaps in part to make room for the GHz Edition.

We've included all three of these cards in this review because they illustrate the current state of the high-end Radeon lineup. Video card makers have more leeway than ever to offer higher-clocked variants of their products, and that means alert enthusiasts can snag some deals by ignoring branding and focusing on specs instead. For example, XFX's "Black Edition" version of the Radeon HD 7950 is so aggressively clocked that it essentially matches the stock 7970 in pixel throughput rate and memory bandwidth. The XFX 7950 does give up a bit of texel fill rate and shader processing oomph to the stock 7970, but we probably wouldn't pay the extra 40 bucks for the 7970, given everything.